PROJECT ABSTRACT In both the United States of America (USA) and the United Kingdom (UK), heart failure (HF) is a common cause of hospitalization, often prolongs length of hospitalization and is associated with high mortality. Although international guidelines on the management of HF are aligned, practice is substantially divergent across countries. There are potentially myriad factors such as variability in healthcare funding, ethnic and racial diversity among others that could be contributing to the variation in practice patterns but it is unclear if this translates into differences in outcomes. We, therefore, propose to evaluate and compare the outcome of patients, using individual patient ?level data (IPLD), with varying levels of evidence for heart failure (HF) in the USA and in the UK. Comparing aggregate high level data for whole population could lead to erroneous conclusions. Access to individual patient-level data (IPLD) enables the outcome of patients with similar attributes in the two countries to be juxtaposed and be analyzed in a uniform manner. As definitions of HF are not robust and diagnostic incentives may differ substantially in the USA and UK, broad criteria for case-ascertainment will be applied to capture the population of interest. This will include both a diagnosis of HF and a prescription of a loop diuretic with a concurrent or previous diagnosis of HF (patients taking loop diuretics have a poor prognosis which may often reflect a missed diagnosis of HF). Spending on Implantable Cardioverter Defibrillator (ICD)/Cardiac Resynchronization Therapy (CRT)/Pacemakers and defibrillators estimated to be c.$5.8 billion in the USA in 2009. However, the efficacies of ICDs and CRTs have been established only in patients selected for clinical trials with relatively few co- morbidities; older people and ethnic minorities were under-represented. Thus, the external validity of trial results is unclear. Comparing practice patterns and clinical outcomes in `real world' patients receiving ICD and CRT across different health systems with widely differing practice might help improve patient selection and management in both countries. There are also doubts about the superiority of CRT-D compared to CRT-P, which is a less expensive technology requiring less sophisticated follow up and is also less likely to malfunction. This would lead to a much more in depth understanding of the two systems and could serve as an input for healthcare policy measures. This ambitious real world project will be conducted in three leading academic research centers ? University Hospitals Cleveland Medical Center/Case Western Reserve University (Cleveland, Ohio, USA), National Heart and Lung Institute/Imperial College (London, England, UK) and Robertson Center for Biostatistics /University of Glasgow (Scotland, UK).